As shown in FIG. 1, a conventional communication frame 100 for a wireless network includes as fields: a preamble 110, a start of frame delimiter (SFD) 120, a physical layer header (PHR) 130, and a physical layer service data unit (PSDU) 140.
The PSDU typically includes a payload, see IEEE 802,15.4a-D7 standard, November 2006, incorporated by reference. The preamble 110 can be used for acquisition and ranging. The SFD 120 is used for frame synchronization. The detection of the SFD indicates the beginning of the PHR 130 and the PSDU 140. The frame 100 has a time duration of Ti 160, and a time duration of the PHR and PSDU is T2 150.
FIG. 2 shows a conventional ranging method. To estimate a distance between a first transceiver A 210 and a second transceiver B 220 in a wireless communications network, the first transceiver A 210 transmits 201 a request frame to the second transceiver B 220. FIG. 2 also shows a time axis 240 for the transceiver A and a time axis 250 for the transceiver B 220. The transceiver A 210 records a time t1 that the request frame was transmitted according to a clock of the transceiver A 210. Upon receiving the request frame, the transceiver B 220 transmits 202 a reply frame to the transceiver A 210. The transceiver A 210 measures the time of arrival (TOA) t2 of the reply frame according to its clock.
A delay at the transceiver B 220 between receiving the request frame and transmitting the reply frame is Ttab 250. The transceiver B 220 transmits 205 the delay 250 to the transceiver A 210 in a timestamp report. An estimate of the distance D between the transceiver A 210 and the transceiver B 220 is the time for the round trip TroundA 270 minus the delay at the transceiver B 220 divided by two and multiplied by the speed of light.
                    D        =                                                            (                                                      t                    2                                    -                                      t                    1                                                  )                            2                        ⁢            c                    =                                                    (                                                      T                    round                    A                                    -                                      T                    ta                    B                                                  )                            2                        ⁢                          c              .                                                          (        1        )            A corresponding one-way flight time Tt 260 is
                              T          t                =                                            (                                                T                  round                  A                                -                                  T                  ta                  B                                            )                        2                    .                                    (        2        )            The conventional method as specified in method in the IEEE 802.15.4a Standard Draft 7 does not address errors due to a frequency offset between the clocks of the transceivers. Indeed, in practical applications, the measurements of TroundA and TtaB are different from their true value due to the frequency offset. The clock tolerance of the transceiver A 210 is eA, and the clock tolerance of the transceiver B 220 as eB.
After factoring in the clock tolerances, the one-way time of flight estimate {circumflex over (T)}t becomes
                                          T            ^                    t                =                                                                              T                  round                  A                                ⁡                                  (                                      1                    +                                          e                      A                                                        )                                            -                                                T                  ta                  B                                ⁡                                  (                                      1                    +                                          e                      B                                                        )                                                      2                    .                                    (        3        )            
A residual error efw is a difference between the time of flight estimate with ideal clocks and with clocks with tolerances eA and eB. Then,etw=TteA+TtaB(eA+eB).  (4)Generally, Tta8>>Tt. Therefore, Equation (4) approximates toetw≈TtaB(eA+eB).  (5)
It is desired to reduce the residual error etw. Therefore, a mechanism is needed to reduce the effect of clock frequency offset on the range error.